In situ remediation options for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in Sydney Harbour sediments

Abstract

Sydney Harbour sediments are severely contaminated with polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) as a result of the intensive production and utilization of organohalide products in Sydney. These highly toxic and recalcitrant contaminants leached into Sydney Harbour with chemical waste that was landfilled on the banks of Homebush Bay and readily accumulated in marine sediments, representing a risk to the ecosystem in Sydney Harbour and human health. In 2017, PCDD/Fs in the harbour sediments were quantified and compared with historical data, with little change being observed over the decade compared with the study of Birch et al. (2007). This suggests that the risks associated with the contamination will remain, until a practical strategy for PCDD/F remediation is developed. Previous studies have shown that microorganisms are able to transform PCDD/Fs, with the potential to detoxify and eliminate these compounds. The goal of this study was to survey the feasibility of applying bioremediation technologies for PCDD/F detoxification in Sydney Harbour sediments. DNA sequencing revealed the presence of bacteria closely related to known PCDD/F degrading microbes belonging to the Dehalococcoides genus in the harbour sediments. Anaerobic enrichment cultures supplied with perchloroethene (PCE) as terminal electron acceptor stimulated the reductive dechlorination of the most toxic dioxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to 2,3,7-trichlorodibenzo-p-dioxin (2,3,7-TriCDD) and the most abundant dioxin octachlorodibenzo-p-dioxin (OCDD) to its hepta- and hexa-chlorinated congeners. This is consistent with the existence of microbes capable of reductively dechlorinating PCDD in Sydney Harbour. The reasons for the lack of significant in situ PCDD/F biotransformation were considered. With 4500 and 10 times higher than the environmental concentrations, respectively, TCDD and OCDD partially but reversibly inhibited the microbial dechlorination activity. The low aqueous solubility of TCDD and OCDD likely limited the capacity of PCDD/F respiring bacteria dechlorinating these compounds and with the use of a biosurfactant lecithin, the microbial dechlorination of TCDD and OCDD was enhanced. These findings demonstrated that applying indigenous microorganisms maybe part of the remedy for Sydney Harbour PCDD/F contamination. Furthermore, this study explored the potential of using a known PCDD dechlorinating bacteria Dehalococcoides mccartyi strain CBDB1 and sulfidized nanoscale zerovalent iron (S-nZVI) in addressing PCDD degradation in conditions found in Sydney Harbour. The tolerance of strain CBDB1 to seawater environments was determined, with a view to its deployment into Sydney Harbour’s contaminated sediments. Additionally, S-nZVI did not react with TCDD and OCDD, but it was capable of supplying H2 and conditioning the redox potential necessary for strain CBDB1 organohalide respiration, with the potential to assist strain CBDB1 dechlorinating PCDD in Sydney Harbour.